The present invention relates, in general, to generating electromagnetic radiation and, more particularly, causing interactions among a plurality of nano-resonant structures to generate electromagnetic radiation.
Electromagnetic radiation or energy is produced by the motion of electrically charged particles including oscillating electrons. For example, when an electron oscillates or is accelerated, varying electric and magnetic fields are generated, thereby emitting electromagnetic waves. The frequency of the emitted electromagnetic wave is a function of the rate the oscillation of the electron. Electromagnetic radiation can be generated across a vast spectrum of frequencies generally categorized as: Radio Frequency is less than 3 Gigahertz, Microwave is 3 Gigahertz to 300 Gigahertz, Infrared is 300 Gigahertz to 400 Terahertz, Visible is 400 Terahertz to 750 Terahertz, Ultraviolet is 750 Terahertz to 30 Petahertz, X-ray is 30 Petahertz to 30 Exahertz, and Gamma-ray is greater than 30 Exahertz.
The structures for generating and detecting electromagnetic radiation generally establish the particular portion of the frequency spectrum the electromagnetic radiation is produced. Structures used to generate higher frequency electromagnetic waves are typically small and more difficult to make. To make higher frequencies, structures are typically made to resonant. For example, klystrons and magnetrons generate microwave electromagnetic waves by using structures having resonate cavities. By further reducing the size of resonant structures, higher frequencies can be achieved. In another example, Smith and Purcell passed electrons over small structures referred to as a grating or a periodically varying metallic surface and generated electromagnetic waves in the visible portion of the electromagnetic spectrum. However, Smith-Purcell where unable to produce electromagnetic wave of sufficient intensity. This is primarily due to electrons being deflected by image charges in the grating. Smith-Purcell devices are inefficient. By increasing the period of the grating beyond the wavelength of the generated electromagnetic waves, Vermont Photonics was able to increase the intensity of the emitted electromagnetic waves.
Devices using resonant structures such as klystrons, backward wave devices, traveling wave tubes and magnetrons can be used to generate electromagnetic waves. Further, the size, structure, and tuning of the resonant cavity or cavities of the devices discussed above establish the characteristic frequency of electron oscillation. In U.S. Pat. No. 6,373,194, Small provides a method for making a micro-magnetron. In U.S. Pat. No. 4,740,973, Madey discloses a free electron laser, which uses relativistic electrons or positron beams and cavities to generate electromagnetic waves. In U.S. Pat. No. 6,909,104, Koops provides a device employing the free-electron laser and a periodic grating without requiring relativistic electrons. A paper by Potylitsin on Apr. 13, 1998 titled “Resonant Diffraction Radiation and Smith-Purcell Effect”, calls for using a resonant diffraction grating. Further, in solid materials the interaction between an electromagnetic wave and a charged particle such as an electron can occur via three basic processes referred to as: absorption, spontaneous emission and stimulated emission. The interaction can provide a transfer of energy between the electromagnetic wave and the electron. For example, photoconductor semiconductor devices use the absorption process to receive the electromagnetic wave and transfer energy to electron-hole pairs by band-to-band transitions. Electromagnetic waves having an energy level greater than a material's characteristic binding energy can create electrons that move when connected across a voltage source to provide a current. In addition, extrinsic photoconductor devices operate having transitions across forbidden-gap energy levels use the absorption process (S. M., Sze, “Semiconductor Devices Physics and Technology”, 2002). A measure of the energy coupled from an electromagnetic wave for a material is referred to as an absorption coefficient. A point where the absorption coefficient decreases rapidly is called a cutoff wavelength. The absorption coefficient is dependent on the particular material used to make a device. For example, gallium arsenide (GaAs) absorbs electromagnetic wave energy from about 0.6 microns and has a cutoff wavelength of about 0.87 microns. In another example, silicon (Si) can absorb energy from about 0.4 microns and has a cutoff wavelength of about 1.1 microns. Thus, the ability to transfer energy to the electrons within the material for making the device is a function of the wavelength or frequency of the electromagnetic wave. This means the device can work to couple the electromagnetic wave's energy only over a particular segment of the terahertz range. At the high end of the terahertz spectrum a Charge Coupled Device (CCD), such as an intrinsic photoconductor device, can successfully be employed. If there is a need to couple energy at the lower end of the terahertz spectrum certain extrinsic semiconductors devices can provide for coupling energy at increasing wavelengths by increasing the doping levels.
Raman spectroscopy is a well-known method to measure the characteristics of molecule vibrations using laser radiation as the excitation source. A molecule to be analyzed is illuminated with laser radiation and the resulting scattered frequencies are collected in a detector and analyzed. Analysis of the scattered frequencies permits the chemical nature of the molecules to be explored. Fleischmann in 1974 first reported the increased scattering intensities that result from Surface Enhanced Raman Spectroscopy (SERS), though without realizing the cause of the increased intensity. In SERS, laser radiation is used to excite molecules adsorbed or deposited onto a roughened or porous metallic surface, or a surface having metallic nano-sized features or structures. The largest increase in scattering intensity is realized with surfaces with features that are 10 100 nm in size. Research into the mechanisms of SERS over the past 25 years suggests that both chemical and electromagnetic factors contribute to the enhancing the Raman effect. (See, e.g., A. Campion and P. Kambhampati, Chem. Soc. Rev., 1998, 27 241.) The electromagnetic contribution occurs when the laser radiation excites plasmon resonances in the metallic surface structures. These plasmons induce local fields of electromagnetic radiation which extend and decay at the rate defined by the dipole decay rate. The local fields contribute to enhancement of the Raman scattering. Surface plasmons can propagate on the surface of a metal as well as on the interface between a metal and dielectric material. Bulk plasmons can propagate beneath the surface, although they are typically not energetically favored. Recent research has shown that changes in the shape and composition of nano-sized features of the substrate cause variation in the intensity and shape of the local fields created by the plasmons. Jackson and Halas (J. B. Jackson and N.J. Halas, PNAS, 2004, 101 17930) used nano-shells of gold to tune the plasmon resonance to different frequencies. Devices using the variation in the local electric field strength provided by the induced plasmon are known as SERS-based devices. In U.S. Patent Application 2004/0174521 A1, Drachev et al. describe a Raman imaging and sensing device employing nano-antennas. The antennas are metal structures deposited onto a surface. The structures are illuminated with laser radiation. The radiation excites a plasmon in the antennas that enhances the Raman scatter of the sample molecule. The electric field intensity surrounding the antennas varies as a function of distance from the antennas, as well as the size of the antennas. The intensity of the local electric field increases as the distance between the antennas decreases. Surface plasmons can be excited at a metal-dielectric interface by a monochromatic light beam. The energy of the light is bound to the surface and propagates as an electromagnetic wave. For more details on all the above application of generating electromagnetic waves or energy see U.S. Pat. No. 7,253,426.
There is a need to improve structures and methods and of generating electromagnetic energy having more intensity. In particular, there is a need to coupling energy from electromagnetic waves in the terahertz range from about 0.1 THz (about 3000 microns) to about 700 THz (about 0.4 microns), which is finding use in numerous new applications. These applications include improved detection of concealed weapons and explosives, improved medical imaging, finding biological materials, better characterization of semiconductors; and broadening the available bandwidth for wireless communications.